U.S. Pat. No. 4,896,149, issued Jan. 23, 1990 ("'149 Patent"), discloses an addressing structure using an ionizable gaseous medium. Such an addressing structure may be used in a system constructed of data storage elements which addresses those data storage elements with the use of an ionizable gas. An example of such a system is a flat panel display, a video camera, or a memory system.
The addressing structure includes an electrode structure having plasma discharge channels for containing the ionizable gaseous medium. Two electrodes extend within and along the length of each channel. Different ones of those two electrodes are operated as a cathode and an anode. When a voltage of predetermined magnitude is applied between the anode and the cathode in a channel, the gaseous medium in that channel ionizes or "fires" and carries an electrical current between that anode and that cathode.
As explained in the '149 Patent, it is desirable for the anode and the cathode in each channel to be connected to the electrical driving circuits at opposite ends of the channel. (See '149 Patent, col. 7, 11. 43-49; col. 12, 11. 8-30; and FIGS. 11A and 11B.) Such a connection of the anode and the cathode tends more nearly to equalize the potential difference between the anode and the cathode along their lengths when they are firing. The plasma resulting from ionization of the ionizable gaseous medium is then more evenly distributed in the channel and thereby promotes better operation of the addressing structure.
It is also desirable to provide in the electrode structure a pathway or channel for removing gas from, and for introducing the ionizable gaseous medium to, each of the channels. One effective way to provide such a pathway is to form in the electrode structure an open region called a "plenum." One end of each of the channels is open to the plenum, which in turn leads to a suitable input/output channel or valve. The plenum facilitates (1) removal of air and introduction of the desired ionizable gaseous medium during manufacture of the electrode structure and (2) purification and/or replenishment of the ionizable gaseous medium from time to time. The plenum is large enough to allow efficient removal of gases from, and addition of gases to, the channels but is not so large as to occupy too much space.
Typically each channel is of the order of 100-200 times as long as the length of the plenum in a direction parallel to each channel. The width of the plenum in a direction perpendicular to each channel is typically equal to the sum of the distance between the two outermost channels of the electrode structure and the width of a gas fill area located at one end of the plenum. (The width of the gas fill area adds about 0.76-1.5 mm to the length of the plenum.) Because there are typically of the order of hundreds to thousands of channels in an electrode structure, the width of the plenum in a direction perpendicular to the length of the channels is typically of the order of hundreds to thousands of times the width of a channel in a direction perpendicular to the length of the channel.
In an electrode structure in which the anodes and cathodes are to be connected to electrical circuitry at opposite ends of the channels and in which a plenum of the type just described is provided, some of the electrodes cross the plenum. In such an electrode structure, a plasma discharge can occur between an electrode fired in the plenum and another electrode in the plenum at ground.
A typical addressing structure system avoids the expense of providing a switch for each electrode. In such a typical addressing structure each electrode that functions as an anode is continuously referenced to ground, and each individual electrode that functions as a cathode is also referenced to ground except during the brief interval when that individual electrode fires in response to a predetermined firing waveform generated by electronic circuitry. If the addressing structure provided for each electrode a switch that disconnected the electrode except when it was supposed to be firing, then any electrodes could cross the plenum. It is, however, expensive to provide such a switch for each electrode.
In the absence of such a switch, if all electrodes were to cross the plenum, a fired cathode would preferably fire in the plenum. To prevent a fired cathode from firing in the plenum, only electrodes which are to be anodes cross the plenum.
Even when only electrodes which are to be anodes cross the plenum, a plasma discharge can nevertheless exist in the plenum as a result of two causes. First, a plasma discharge in a channel between a fired cathode and an anode located in that channel can spread outside that channel along the length of either that fired cathode or that anode. Second, a plasma discharge in a channel initiated by a fired cathode can extend to either an anode or a cathode at ground in another channel.
A plasma discharge which extends into the plenum can have any one or more of several adverse effects.
First, a plasma discharge in the plenum is likely to carry much more current per unit length of the electrodes involved in the discharge than a plasma discharge in a channel. The walls of the channel and of the plenum act as quenching agents to limit the intensity of a plasma discharge by attracting and/or capturing ions, and/or by neutralizing ions, and/or by promoting neutralization among ions having charges of opposite polarity. However, in the channels the ratio of wall surface to gas volume enclosed is much higher than in the plenum. The walls of the plenum are farther from most of the volume of the plenum than the walls of the channels are from most of the volume of the channels. A plasma discharge in the plenum can thus attract so much of the current flowing between the electrodes involved that the ionizable gaseous medium may not become or remain ionized in a channel in which it was supposed to become or remain ionized. Failure of ionization in a channel will prevent proper operation of the addressing structure.
Second, if the current flowing through a plasma discharge in the plenum is large enough, the involved electrodes may be eroded. (An anode may be eroded by arcing, and a cathode may be eroded by arcing or sputtering.) Undue electrode erosion typically occurs where the current per unit length of the electrode passing between the electrode and the ionizable gaseous medium is much larger than the current per unit length for which the electrode is designed, or is much larger than the current per unit length of other electrodes. In addition, the perimeter seal of the plenum may be eroded, or the material of which the perimeter seal is made may be disassociated, raising the possibility of degradation of the ionizable gaseous medium through leaks in the perimeter seal or through introducing contaminates into the ionizable gaseous medium.
Third, a plasma discharge in the plenum may generate enough ions for the ions to migrate into one or more channels which are not supposed to have ions in them at the time the ions migrate in. Such migrating ions can neutralize the charge controlling the state of memory of display cells of the addressing structure, thereby degrading the operation of the addressing structure.
There is thus a need to find some way to reduce or eliminate plasma discharge in the plenum while permitting the desired plasma formation in the channels.